With the advent of an advanced information society, there has recently been an enormous increase in amount of information handled. Accordingly, there has been a need for increases in storage capacity and density of a recording apparatus. In the field of magnetic recording typified by hard disks, a recording density exceeding an area density of 300 Gbit/inch2 is expected to be achieved very soon through improvements in characteristics of a recording medium, a recording head, and a reproducing head. Improvements in area density of a recording medium are still under way. It is considered that the area density will exceed 1 Tbit/inch2 in the future.
In order to increase the density of a magnetic recording medium, it is necessary to miniaturize individual magnetic particles that form a magnetic bit on a magnetic recording medium. Unfortunately, a magnetic recording medium having miniaturized magnetic particles is easily affected by external thermal energy. This leads to a problem of thermal fluctuation. The problem of thermal fluctuation refers to a difficulty caused by external thermal energy in stably retaining magnetic information for a long period. The difficulty is caused in such a way that the effect of external thermal energy makes it impossible to maintain a magnetic axis in one orientation, and this causes a difficulty in stably retaining a recorded magnetization.
The effect of thermal fluctuation is determined by a correlation between magnetic anisotropic energy Ku·V (Ku: magnetic anisotropic energy; V: volume of a magnetic particle) and thermal energy kT (k: Boltzmann constant; T: temperature in a medium), which disrupts a recorded state. Specifically, the effect of thermal fluctuation becomes significant when the magnetic anisotropic energy Ku·V is decreased to about dozens of times the thermal energy kT. For this reason, in order to lessen the effect of thermal fluctuation, for example, in the case of a decrease in the volume V of the magnetic particle, it is necessary to increase the magnetic energy of the magnetic particle by adopting a material high in Ku.
As magnetic recording media that solve such a problem of thermal fluctuation and increase recording density, patterned media are known (see, for example, Patent Literature 1 and Patent Literature 2). A patterned medium is arranged such that individual magnetic particles are separated from one another by separators each made of a nonmagnetic body and one magnetic particle forms one magnetic bit, whereas a granular medium, which is a magnetic recording medium used as a conventional hard disk, is arranged such that a plurality of small magnetic particles form one magnetic bit. This is effective against the problem of thermal fluctuation because this makes it possible to increase the volume V of a magnetic particle of a patterned medium as compared to a granular medium.
Patent Literature 1 discloses a method for manufacturing a patterned medium including the steps of: forming, on a substrate, a continuous or discontinuous groove area or a zonal area containing a specific chemical component, which groove area or zonal area corresponds to a recording track zone; forming a two-dimensional regular array of self-organizing molecules or fine particles; and forming recording cells corresponding to the regular array. Patent Literature 2 discloses, as an example of a method for forming a magnetic recording layer of a patterned medium, the formation of a film of magnetic material on a predetermined substrate and the subsequent etching of the film of magnetic material via a predetermined mask. Patent Literature 2 also describes the use as a masking material of silver (Ag), chrome (Cr), tungsten (W), molybdenum (Mo), tantalum (Ta), or an alloy containing the metals as its main component, which metal or alloy is deposited in an island shape.
Media made with the aim of an increase in density of a magnetic recording medium as is the case with patterned media encompass discrete media. A discrete medium is a magnetic recording medium arranged such that magnetic tracks are separated from one another by separators each made of a nonmagnetic body. Patent Literature 3 describes the use of a discrete medium as an art for resolving a problem of cross write on adjacent magnetic tracks. This makes it possible to prevent a fringing from causing a recording bit to spread in a track width direction.
On the other hand, an optically-assisted magnetic recording method has been proposed as a recording method that makes it possible to record magnetic bits with high spatial resolution on a magnetic recording medium. The optically-assisted magnetic recording method, which attracts attention as a promising art for next-generation high-density magnetic recording, is a method in which magnetic recording is performed on a magnetic recording medium high in both coercitivity (Hc) and resistance to thermal fluctuation. Specifically, the optically-assisted magnetic recording method is a recording method in which the coercitivity (Hc) of a magnetic recording medium is decreased by converging light on a surface of the magnetic recording medium and thus locally raising the temperature of the magnetic recording medium and magnetic recording is performed on the magnetic recording medium by applying an external magnetic field to an area having been decreased in the coercitivity (Hc). Patent Literature 4 discloses a method in which information is recorded by the optically-assisted magnetic recording method on a magnetic recording medium having a recording layer made of a ferrimagnetic body.
The optically-assisted magnetic recording method makes it possible to record information, by use of a relatively small magnetic field, even onto a magnetic recording medium made of a material high in magnetic anisotropic energy (Ku). This makes it possible to stably retain magnetic information and prevent loss of recorded information even in the case of a high-density magnetic recording medium that requires high magnetic anisotropic energy (Ku).
Furthermore, the optically-assisted magnetic recording method makes it possible to determine the size of a recording bit in accordance with the size of a heated area on a magnetic recording medium. Therefore, a reduction in the heated area on the magnetic recording medium makes it possible to form a minute recording bit, even if an area to which an external magnetic field is applied is large. This makes it possible to achieve magnetic recording further higher in density.
Each of Patent Literatures 5 through 7 discloses a method in which near-field light is used as a heat source for locally heating a magnetic recording medium. The near-field light here refers to light (electromagnetic field) that is generated by causing light to come into a microstructure smaller than a wavelength of light, e.g., a structure such as an aperture, and is localized only in close proximity to the aperture. The near-field light generated in the vicinity of the aperture does not propagate to another area, but stays in close proximity of the aperture.
Patent Literature 5 describes: an optically-assisted magnetic recording apparatus whose light irradiating means for irradiating a magnetic recording layer with near-field light and heating an area thus irradiated is an optical waveguide or an optical probe; and an example of application of the optically-assisted magnetic recording apparatus to a discrete medium. Patent Literature 6 discloses: a probe utilizing a planar scatterer that lessens the effect of near-field light generated at a position other than a point at which intense near-field light is generated; and an example of application of the probe to a recording/reproducing apparatus. Patent Literature 7 discloses: an electromagnetic field generating element that can (i) generate a strong near field in the vicinity of an edge of a conductor by irradiating the conductor with laser light, and, on the other hand, (ii) generate a magnetic field around the conductor by passing an electric current through the conductor; and an example of the application of the electromagnetic field generating element to an information recording/reproducing apparatus.
As described above, changing a recording medium and/or a recording method is under consideration for an increase in density of a magnetic recording medium. In particular, in a case where a patterned medium or a discrete medium and the optically-assisted magnetic recording method are combined as a magnetic recording medium and a recording method, respectively, it is possible to realize a magnetic recording medium higher in density, as compared to a case where each of the arts is solely adopted.